Nonaqueous secondary battery, method for making negative electrode component therefor, and apparatuses for evaluating and making graphite material for negative electrode component

ABSTRACT

A negative electrode of a nonaqueous secondary battery is formed of a carbonaceous material. The ratio RG=Gs/Gb of the degree of graphitization Gs of the carbonaceous material, determined by a surface-enhanced Raman spectrum, to the degree of graphitization Gb, determined by a Raman spectrum measured using argon laser light, is at least 4.5. Alternatively, the carbonaceous material has a peak in a wavelength range above 1,360 cm −1  in a surface-enhanced Raman spectrum which is measured by the same surface-enhanced Raman spectrum. The deterioration of the nonaqueous secondary battery is suppressed during use in high-temperature environments and high capacity is maintained for long periods.

The present application is a divisional of U.S. patent application Ser.No. 09/514,590 filed Feb. 28, 2000 now U.S. Pat. No. 6,506,519.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a secondary battery using a nonaqueouselectrolyte solution (hereinafter referred to as a “nonaqueous secondarybattery”), a method for making a negative electrode component used inthe nonaqueous secondary battery, an apparatus for evaluating a graphitematerial for the negative electrode component, and an apparatus formaking the graphite material.

2. Description of the Related Art

As rapid progress is made in the miniaturization of electronic devices,such as portable phones and notebook personal computers, secondarybatteries are required to have higher energy densities.

In conventional secondary batteries, such as lead batteries, Ni—Cdbatteries, and Ni-MH batteries, discharge voltages are low and energydensities are insufficient. Lithium secondary batteries are also used inpractice, in which metallic lithium, lithium alloys, and carbonaceousmaterials which can electrochemically occlude and release lithium ionsare used as negative electrode active materials, and various positiveelectrodes are used. The lithium secondary batteries have high outputvoltages and thus have large energy densities per weight or volumecompared to the above conventional batteries.

In the lithium secondary batteries at initial stages, metallic lithiumand lithium alloys are used as negative electrodes. A negative electrodeusing metallic lithium or a lithium alloy is insufficient incharge-discharge efficiency and has a problem in that dendritic lithiumis formed. Thus, such negative electrodes are used in practice only in afew specialized fields.

Carbonaceous materials which can electrochemically occlude and releaselithium ions have recently been anticipated as negative electrodecomponents and are now coming into use. Negative electrodes using thecarbonaceous materials do not have problems inherent in the metalliclithium or lithium alloys, that is, the formation of metallic lithiumhaving a dendritic structure and particularization of the lithium alloyduring charge-discharge cycles. Moreover, the carbonaceous materialsshow high coulomb efficiency; hence, lithium secondary batteries havingcarbonaceous negative electrodes exhibit superior charge-dischargereversibility.

In secondary batteries using the carbonaceous materials as negativeelectrode active materials, metallic lithium is not precipitated in use.Thus, lithium secondary batteries using the carbonaceous materials andnonflammable lithium compound oxide are safe and are commerciallyproduced. These batteries are called “lithium ion batteries” and usecarbonaceous materials as negative electrodes, LiCoO₂ as positiveelectrodes, and nonaqueous electrolyte solutions containing nonaqueoussolvents.

Carbonaceous materials used as negative electrodes are classified intographite materials including natural products and artificial products,easily-graphitizable carbonaceous materials as precursors of artificialgraphite materials, and ungraphitizable carbonaceous materials which arenot converted to graphite even when these are treated at hightemperatures facilitating the formation of graphite. Graphite materialsand ungraphitizable carbonaceous materials have high capacities asnegative electrodes and are thus currently used.

Lithium ion batteries have rapidly gained widespread use as electricalpower sources in electronic devices, particularly, notebook personalcomputers due to compact, because they are lightweight, and have highcapacities. Notebook personal computers having improved performancerequire higher,CPU clock frequencies. Thus, high-performance computersconsume significant amounts of electrical power and generate substantialamounts of heat during operation. Moreover, the restricted volume ofdead space, which is inherent in miniaturization of personal computers,precludes the dissipation of heat which is generated during operation,resulting in an increase in the internal temperatures of personalcomputers.

The increased temperature accelerates deterioration and thus decreasescapacity in batteries used in personal computers. The lost capacitycannot be restored by any means.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide alithium ion secondary battery which does not cause deterioration bygenerating excessive heat, has high capacity, and is highly reliable inuse.

It is another object of the present invention to provide a method formaking a negative electrode component used in a nonaqueous secondarybattery, an apparatus for evaluating a graphite material as the negativeelectrode component, and an apparatus for making the graphite material.

According to experimental results obtained by the present inventors, alithium ion secondary battery using a carbonaceous material for anegative electrode, which has specific structural parameters, canmaintain high capacity even after such a battery is stored inhigh-temperature environments, and has high capacity reliability forlong periods.

According to a first aspect of the present invention, a nonaqueoussecondary battery includes a negative electrode including a carbonaceousmaterial in which the ratio RG=Gs/Gb of the degree of graphitization Gsof the carbonaceous material, determined by a surface-enhanced Ramanspectrum, to the degree of graphitization Gb, determined by a Ramanspectrum measured using argon laser light, is at least 4.5, based on thefollowing conditions:Gb=Hbb/Hba; andGs=Hsb/Hsa;

wherein Hba is the height of a peak lying in a range of 1,580 cm⁻¹ to1,620 cm⁻¹ in a Raman spectrum which is measured using an argon laserRaman spectrometer of a wavelength of 514.5 nm and a wavelengthresolution of 4 cm⁻¹;

Hbb is the height of a peak lying in a range of 1,350 cm⁻¹ to 1,400 cm⁻¹in a Raman spectrum which is measured using an argon laser Ramanspectrometer of a wavelength of 514.5 nm and a wavelength resolution of4 cm⁻¹;

Hsa is the height of a peak lying in a range of 1,580 cm⁻¹ to 1,620 cm⁻¹in a surface-enhanced Raman spectrum which is measured using an argonlaser Raman spectrometer of a wavelength of 514.5 nm and a wavelengthresolution of 4 cm⁻¹ when silver having a thickness of 10 nm isdeposited on the carbonaceous material; and

Hsb is the height of a peak lying in a range of 1,350 cm⁻¹ to 1,400 cm⁻¹in a surface-enhanced Raman spectrum which is measured using an argonlaser Raman spectrometer of a wavelength of 514.5 nm and a wavelengthresolution of 4 cm⁻¹ when silver having a thickness of 10 nm isdeposited on the carbonaceous material.

According to a second aspect of the present invention, a nonaqueoussecondary battery includes a negative electrode including a carbonaceousmaterial having a peak in a wavelength range above 1,360 cm⁻¹ in asurface-enhanced Raman spectrum which is measured using an argon laserRaman spectrometer of a wavelength of 514.5 nm and a wavelengthresolution of 4 cm⁻¹ when silver having a thickness of 10 nm isdeposited on the carbonaceous material.

In the nonaqueous secondary battery according to the first aspect andsecond aspect, the carbonaceous material is preferably graphite.

The nonaqueous secondary battery further includes a positive electrode,which preferably includes a lithium compound oxide represented byLiM_(x)O_(y) wherein M is at least one element selected from the groupconsisting of Co, Ni, Mn, Fe, Cr, Al, and Ti.

According to a third aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the first aspect, includes the steps of carbonizinga raw material, slightly oxidizing the surface of the carbonizedmaterial, and then graphitizing the oxidized material.

According to a fourth aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the second aspect, includes the steps ofcarbonizing a raw material, slightly oxidizing the surface of thecarbonized material, and then graphitizing the oxidized material.

According to a fifth aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the first aspect, includes the step of polishingthe surface of the negative electrode component by irradiating thesurface with light.

According to a sixth aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the second aspect, includes the step of polishingthe surface of the negative electrode component by irradiating thesurface with light.

According to a seventh aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the first aspect, includes the step of annealingnatural graphite having a rhombic structure at a temperature of at least2,000° C.

According to an eighth aspect of the present invention, a method formaking a negative electrode component used in the nonaqueous secondarybattery according to the second aspect, includes the step of annealingnatural graphite having a rhombic structure at a temperature of at least2,000° C.

According to a ninth aspect of the present invention, an apparatus forevaluating a graphite material includes a surface-enhanced Ramanspectroscopic means.

According to a tenth aspect of the present invention, an apparatus forevaluating a graphite material used in a nonaqueous secondary battery,determines whether the ratio RG=Gs/Gb of the degree of graphitization Gsof the carbonaceous material, determined by a surface-enhanced Ramanspectrum, to the degree of graphitization Gb, determined by a Ramanspectrum measured using argon laser light, is at least 4.5, based on theconditions described in the first aspect.

According to an eleventh aspect of the present invention, an apparatusfor evaluating a graphite material used in a nonaqueous secondarybattery, determines if the graphite material has a peak in a wavelengthrange above 1,360 cm⁻¹ in a surface-enhanced Raman spectrum which ismeasured using an argon laser Raman spectrometer of a wavelength of514.5 nm and a wavelength resolution of 4 cm⁻¹ when silver having athickness of 10 nm is deposited on the carbonaceous material.

According to a twelfth aspect of the present invention, an apparatus formaking a graphite material includes an apparatus for evaluating agraphite material according to the ninth aspect.

According to a thirteenth aspect of the present invention, an apparatusfor making a graphite material includes an apparatus for evaluating agraphite material according to the tenth aspect.

According to a fourteenth aspect of the present invention, an apparatusfor making a graphite material includes an apparatus for evaluating agraphite material according to the eleventh aspect.

The method for measuring the structural parameters characterized by thepresent invention is described below. The structural parameters aredetermined by Raman spectroscopy. The Raman spectrum of a conventionalcarbonaceous material has a peak Pba in a range of 1,580 cm⁻¹ to 1,620cm⁻¹ assigned to a graphite crystal structure and a peak Pbb in a rangeof 1,350 cm⁻¹ to 1,400 cm⁻¹ assigned to an amorphous structure. When thegraphite structure is disordered, the intensity Hba of the peak Pbadecreases, whereas the intensity Hbb of the peak Pbb increases. Thus,the ratio of the height Hba to the height Hbb represents the degree ofgraphitization.

Surface-enhanced Raman spectrometry (SERS) was developed by Fleischmannet al. in 1974. In this method, a thin metal layer is deposited on thesurface of a sample to be measured. This method is characterized byuppermost surface analysis on the order of several nanometers andenhanced Raman sensitivity. A surface-enhanced Raman spectrum issubstantially the same as the corresponding conventional Raman spectrum,although the analytical depth differs between these methods. That is,the surface-enhanced Raman spectrum has a peak Pba in a range of 1,580cm⁻¹ to 1,620 cm⁻¹ assigned to a graphite crystal structure and a peakPbb in a range of 1,350 cm⁻¹ to 1,400 cm⁻¹ assigned to an amorphousstructure. The ratio of the intensity (height Hba) of the peak Psa tothe intensity (height Hbb) of the peak Psb represents the degree ofgraphitization at the topmost layer.

The carbonaceous material for the negative electrode component in thenonaqueous secondary battery in accordance with the present invention ischaracterized by the ratio of the intensities measured by the two typesof Raman spectroscopy, and the capacity of the battery can be maintainedeven after the battery is stored in high-temperature environments.

Such a carbonaceous material can be produced by the method for makingthe negative electrode component in accordance with the presentinvention.

The apparatus for evaluating the graphite material including asurface-enhanced Raman spectrometer can evaluate the graphite materialbefore a battery is fabricated using the graphite material.

The apparatus for making the graphite material including the apparatusfor evaluating the graphite material contributes to improved qualitycontrol in the manufacturing process, optimization of the process, andincreasing the rate of development of the graphite material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cylindrical nonaqueous secondarybattery containing a spiral electrode element in accordance with thepresent invention;

FIG. 2 is a graph of the relationship between the RG ratio and thecapacity recovery rate; and

FIG. 3 is a graph of the relationship between the Psb value and thecapacity recovery rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Any carbonaceous material which satisfies the above-described structuralparameters can be used in the present invention. Preferred carbonaceousmaterials in the present invention are graphite materials which areclassified into natural graphite derived from ore and artificialgraphite which is formed by heating an organic material to a hightemperature of at least 2,000° C.

Examples of organic materials include coal and pitch. Examples ofpitches include tars which are formed by high-temperature pyrolysis ofcoal tar, ethylene bottom oil, and crude oil; products which are formedby distillation (vacuum distillation, normal-pressure distillation, andsteam distillation), thermal condensation polymerization, extraction,and chemical condensation polymerization of asphalt; and pitches formedby coking wood. Examples of starting materials for pitch includepolyvinyl chloride resins, polyvinyl acetate resins, polyvinyl butyralresin, and 3,5-dimethylphenyl resins. The coal and pitch are present ina liquid state at up to 400° C. in the carbonization process, aromaticrings are polymerized, fused, and stacked at this temperature, and thensemicokes as precursors of carbonaceous materials are formed at atemperature of 500° C. or more. This process is called liquid-phasecarbonization and is a typical process for forming graphitizable carbon.

Examples of other useful starting materials include fused-ringpolycyclic compounds, e.g., naphthalene, phenanthrene, anthracene,triphenylene, pyrene, perylene, pentaphene, pentacene, and derivativesthereof, e.g., carboxylic acids, carboxylic anhydrides, and carboxylicimides thereof; fused-ring heterocyclic compounds, e.g., acenaphthylene,indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine,carbazole, acridine, phenazine, phenanthridine, and derivatives thereof.

A desired artificial graphite can be produced using one of the aboveorganic materials as a starting material, for example, as follows. Theorganic material is carbonized at 300 to 700° C. in a stream of an inertgas, such as nitrogen, is heated to 900 to 1,500° C. at a heating rateof 1 to 100° C./min in the inert gas stream, is calcined at thistemperature for 0 to 30 hours, and is then subjected to high-temperatureheat treatment at a temperature of at least 2,000° C. and preferably2,500° C. Carbonization and calcination may be omitted in some cases.The resulting carbonaceous or graphitic materials are pulverized andthen classified before the material is used in a negative electrodecomponent. Pulverization is preferably performed prior to carbonization,calcination, and high-temperature heat treatment.

A material which is preferable for use in practice is a graphitematerial having a true density of at least 2.1 g/cm³ and a bulk densityof at least 0.4 g/cm³. Since the graphite material has a high truedensity, a negative electrode using the graphite material has a highpackaging density, resulting in improved energy density when used inbatteries. When a graphite material is used having a bulk density of atleast 0.4 g/cm³, the graphite material is homogeneously dispersed in anegative electrode compound, and thus cycle characteristics areimproved. When a graphite material is used having a bulk density of 0.4g/cm³ and a high flatness represented by an average shape factor x_(ave)of 125 or less, the electrode has an ideal structure and exhibitssatisfactory cycle characteristics.

In order to produce such a graphite material, a molded carbon article ispreferably subjected to the heat treatment for graphatization. Thisgraphitized molded article is pulverized to form a graphite materialhaving a higher bulk density and a lower average shape factor x_(ave).When the pulverized graphite material has a bulk density of 9 m²/g orless in addition to the above parameters, the number of fine submicronparticles adhering onto graphite particles is reduced. Thus, the bulkdensity is increased, and the electrode has an ideal structureexhibiting further improved cycle characteristics.

The reliability of the nonaqueous secondary battery is further improvedwhen pulverized graphite is used having a 10% cumulative particle sizeof 3 μm, a 50% cumulative particle size of 10 μm, and a 90% cumulativeparticle size of 70 μm according to a particle size distribution bylaser diffractometry.

Small particles have large specific areas and cause extraordinaryamounts of heat generation when the secondary battery is overcharged.Since the amount of particles of small size is restricted in the presentinvention, the extraordinary heat generation does not occur in anovercharged state. Also, the amount of particles of large size is alsorestricted in the present invention, hence, expansion of particles atthe initial charging operation can be suppressed. Thus, the nonaqueoussecondary battery does not cause internal short-circuiting due to theexpansion of particles, and is very safe and reliable in practical use.

The use of graphite particles having an average fracture strength of atleast 6.0 kgf/mm² causes the formation of numerous pores which cancontain the electrolyte solution in the electrode formed of thegraphite. As a result, the nonaqueous secondary battery exhibitssatisfactory load characteristics.

A method for making the above-described carbonaceous material inaccordance with the present invention will now be described.

The production of artificial graphites is described below. Various typesof pitch formed from petroleum and coal can be used as raw materials forprecursors of the artificial graphite. The pitch is heated to 400 to500° C. to form a carbonaceous precursor, and the resulting precursor isheated to 800 to 1,000° C. in an atmosphere of inert gas. The product ispulverized and classified so as to have the above-specified particlesize. The resulting particles are heated to a predetermined temperaturein an atmosphere of reactive gas to slightly oxidize the surfacesthereof and are then heated to a higher temperature in an atmosphere ofinert gas to graphitize the particles. The artificial graphite inaccordance with the present invention is thereby formed.

Any reactive gas reactive with carbon may be used. Examples of preferredreactive gases include oxygen, ozone, carbon dioxide, chlorine,hydrochloric acid, sulfur dioxide, and NO_(x). The reaction temperaturecan be determined according to the type of gas used and is preferablyroom temperature to 500° C. The graphitized particles are preferablyirradiated with intense light such as laser light to remove or polishthe surface unevenness of the particles so that the artificial graphitesatisfies the above-described requirements.

The processing of natural graphite will be described below. When thenatural graphite is pulverized under appropriate conditions, a rhombiccrystal structure is formed. The pulverized graphite is heated to 2,000°C. or more to form the carbonaceous material in accordance with thepresent invention. The content of the rhombic structure can bedetermined by X-ray diffractometry and is in a range of preferably 1% to40% and more preferably 5% to 30%.

The materials for the positive electrode used in a combination of theabove negative electrode preferably contain a sufficient amount of Li.Examples of preferable materials include lithium compound oxidesrepresented by the general formula LiMO₂ wherein M is at least onetransition metal selected from the group consisting of Co, Ni, Mn, Fe,Al, V, and Ti, and interlayer compounds containing lithium.

The nonaqueous electrolyte solution used in the nonaqueous secondarybattery in accordance with the present invention is composed of anelectrolyte and nonaqueous solvents. In the present invention, thenonaqueous solvents are composed of a first solvent having a relativelyhigh dielectric constant, such as ethylene carbonate (EC), and anadditional solvent as a low-viscosity component.

Examples of the first solvents having high dielectric constants includepropylene carbonate (PC), butylene carbonate, vinylene carbonate,sulfolane and derivatives thereof, butyrolactone and derivativesthereof, and valerolactone and derivatives thereof.

Examples of preferred low-viscosity solvents include symmetric orasymmetric linear carbonate esters, e.g., diethyl carbonate, dimethylcarbonate, methyl ethyl carbonate, and methyl propyl carbonate. Acombination of at least two low-viscosity solvents is more preferable.

When the graphite material is used in the negative electrode, preferredfirst nonaqueous solvents are EC and halogenated EC. It is preferablethat a second high-dielectric solvent, such as PC, be partiallysubstituted for EC and/or halogenated EC in order to further improvecharacteristics of the nonaqueous secondary battery, even though PC isreactive with the graphite material.

Examples of the second high-dielectric solvents include PC, butylenecarbonate, 1,2-dimethoxyethane, 1,2-diethoxydrofuran, γ-butyrolactone,valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane,4- methy-1,3-dioxolane, sulfolane, and methylsulfolane. The content ofthe second high-dielectric solvents is preferably less than 10 percentby volume.

In the present invention, a third solvent may be added to the firstsolvent which may contain the second solvent, as described above, inorder to improve conductivity and low-temperature characteristics, tosuppress decomposition of EC, and to suppress reactivity of the solventmixture with lithium so as to ensure the safety of the secondarybattery. Examples of preferred third solvents are linear carbonateesters, such as diethyl carbonate (DEC) and dimethyl carbonate (DMC);and asymmetric linear carbonate esters, such as methyl ethyl carbonate(MEC) and methyl propyl carbonate (MPC).

The ratio by volume of the third solvent to the first solvent which maycontain the second solvent, that is, (the first solvent+optional secondsolvent):(third solvent) is preferably 15:85 to 40:60 and morepreferably 18:82 to 35:65. The third solvent may be a mixture of MEC andDMC. The MEC-DMC ratio in the mixed third solvent is preferably in arange of 1/9≦d/m≦8/2 wherein d represents the volume of MEC and mrepresents the volume of DMC.

When the third solvent is a mixture of MEC and DMC, the amount (m+d) ofthe third solvent in the total amount T of the solvents is preferably ina range of 3/10≦(m+d)/T≦9/10 and more preferably 5/10≦(m+d)/T≦8/10.

Any electrolytes used in secondary batteries may be used together withthe above nonaqueous solvents in the present invention. A preferredelectrolyte is LiPF₆. Examples of other usable electrolytes includeLiClO₄, LiAsF₆, LiBF₄, LiB(C₆H₅)₄, CH₃SO₃Li, CF₃SO₃Li, LiN(CF₃SO₂)₂,LiC(CF₃SO₂)₃, LiCl, and LiBr. These electrolytes may be used alone or incombination.

Examples of the nonaqueous secondary battery in accordance with thepresent invention will now be described with reference to FIG. 1. Ofcourse, the present invention is not limited to these Examples.

EXAMPLE 1

A negative electrode 1 was prepared as follows. Coal pitch and petroleumpitch were mixed and were then formed under pressure. The product washeated to 500° C. in an atmosphere of inert gas and was then pulverized,and then the pulverized product was classified. The classified particleswere heated to 1,000° C. in an atmosphere of inert gas to form agraphite precursor. The precursor was heated to 200° C. for 20 hours ina hermetically sealed oxygen chamber, and was then heated to 2,950° C.for 1 hour in an atmosphere of inert gas.

A mixture of 90 parts by weight of the resulting carbonaceous particlesand 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder wasprepared as a negative electrode compound. The compound was dispersedinto N-methylpyrrolidone to form a slurry or paste. The slurry wasapplied onto two faces of a copper foil strip having a thickness of 10μm as a negative electrode collector, was dried, and was subjected tocompact molding to form a strip negative electrode 1.

A positive electrode 2 was prepared as follows. A mixture of 0.5 mole oflithium carbonate and 1 mole of cobalt carbonate was baked at 900° C.for 5 hours in air to form LiCoO₂ as a positive electrode activematerial. A positive electrode compound was prepared by mixing 91 partsby weight of LiCoO₂, 6 parts by weight of graphite as a conductiveagent, and 3 parts by weight of polyvinylidene fluoride as a binder. Thepositive electrode compound was dispersed into N-methylpyrrolidone toform a slurry or paste. This slurry was uniformly applied onto two facesof an aluminum foil strip having a thickness of 20 μm as a collector,was dried, and was subjected to compact molding to form a strip positiveelectrode 2.

The negative electrode 1, a separator 3 composed of a microporouspolypropylene film having a thickness of 25 μm, and then the positiveelectrode 2 were laminated in that order, and the laminate was woundaround a center pin 14 by a plurality of turns. The outermost end of theseparator 3 was fixed with a tape to form a spiral electrode element.

The resulting electrode element was placed into a battery can 5 in whichinsulating plates 4 were arranged above and below the electrodeelements. A positive electrode lead 13 covered with insulating films wasextracted from a positive electrode collector 11 and was welded to asafety valve 8 which is connected to the battery lid 7. A negativeelectrode lead 12 was extracted from a negative electrode collector 10and was welded to the battery can 5. The battery can 5 was made of ironand had an outer diameter of 18 mm, an inner diameter of 17.38, athickness of 0.31 mm, and a height of 65 mm.

The battery can 5 was filled with an electrolyte solution of LiPF₆ in anequivolume mixture of ethylene carbonate and dimethyl carbonate in aratio of 1 mole/liter. The battery can 5 was caulked with a gasket 6 tofix the battery lid 7. A nonaqueous secondary battery having ahermetically sealed structure was thereby prepared.

EXAMPLE 2

Acenaphthylene pitch was heated to 400° C. under a pressure of at least10 kg/cm² so that a bulk mesophase formed. The product was heated to500° C. in an atmosphere of inert gas and was pulverized, and then thepulverized product was classified. The classified particles were heatedto 1,000° C. in an atmosphere of inert gas to form a graphite precursor.The graphite precursor was heated to 3,050° C. for 1 hour in anatmosphere of inert gas. Using this product as a negative electrodecomponent, a nonaqueous secondary battery was prepared as in Example 1.

EXAMPLE 3

A small amount of sulfuric acid was added to acenaphthylene and themixture was heated to 400° C. under a pressure of at least 10 kg/cm² sothat a bulk mesophase formed. The product was heated to 500° C. in anatmosphere of inert gas and was pulverized, and then the pulverizedproduct was classified. The classified particles were heated to 1,000°C. in an atmosphere of inert gas to form a graphite precursor. Thegraphite precursor was heated to 3,050° C. for 1 hour in an atmosphereof inert gas. Using this product as a negative electrode component, anonaqueous secondary battery was prepared as in Example 1.

EXAMPLE 4

A purified natural graphite having a purity of at least 99% waspulverized using a ball mill and was then classified. The pulverizedgraphite was agitated and was simultaneously irradiated with laser lightin air. The graphite was then heated to 2,600° C. for 1 hour in anatmosphere of inert gas. Using this product as a negative electrodecomponent, a nonaqueous secondary battery was prepared as in Example 1.

EXAMPLE 5

A purified natural graphite having a purity of at least 99% waspulverized using a jet mill, and was simultaneously classified bywinnowing using an air jet. The pulverized graphite comprised 20%rhombic structures, according to X-ray diffractometry. The pulverizedgraphite was heated to 2,700° C. for 1 hour in an atmosphere of inertgas. Using this product as a negative electrode component, a nonaqueoussecondary battery was prepared as in Example 1.

COMPARATIVE EXAMPLE 1

After mixing coal pitch and petroleum pitch, the mixture was formedunder pressure. The product was heated to 500° C. in an atmosphere ofinert gas and was pulverized, and then the pulverized product wasclassified. The classified particles were heated to 1,000° C. in anatmosphere of inert gas to form a graphite precursor. The graphiteprecursor was heated to 2,950° C. for 1 hour in an atmosphere of inertgas. Using this product as a negative electrode component, a nonaqueoussecondary battery was prepared as in Example 1.

COMPARATIVE EXAMPLE 2

Acenaphthylene pitch was heated to 400° C., and when the content ofmesophase microspheres reached 50%, these microspheres were isolatedfrom the matrix. The microspheres were heated to 500° C. in anatmosphere of inert gas and was pulverized, and then the pulverizedproduct was classified. The classified particles were heated to 1,000°C. in an atmosphere of inert gas to form a graphite precursor. Thegraphite precursor was heated to 2,900° C. for 1 hour in an atmosphereof inert gas. Using this product as a negative electrode component, anonaqueous secondary battery was prepared as in Example 1.

COMPARATIVE EXAMPLE 3

A purified natural graphite having a purity of at least 99% waspulverized using a ball mill and was then classified. Using this productas a negative electrode component, a nonaqueous secondary battery wasprepared as in Example 1.

The batteries of Examples 1 to 5 and Comparative Examples 1 to 4 weredischarged under conditions of a maximum voltage of 4.2 volts, aconstant current of 1 ampere, and a charging time of 3 hours. Thecharged batteries were stored in an atmosphere at 45° C. for one month,in consideration of the internal temperature of electronic devicesduring operation. The batteries were discharged to measure dischargecapacities thereof and were then charged under the above conditions. Thecharged batteries were discharged at a discharge current of 1 ampere todetermine discharge capacities thereof as recovery capacities. A valueof the recovery capacities divided by the discharge capacities afterstorage was defined as the capacity recovery rate, which is shown inTable 1.

TABLE 1 Capacity Recovery RG Pbb Pba Psb Psa Rate (%) Ratio (cm⁻¹)(cm⁻¹) (cm⁻¹) (cm⁻¹) Example 1 91.2 5.38 1352.70 1574.81 1379.17 1604.00Example 2 94.9 7.56 1353.84 1576.90 1388.50 1581.22 Example 3 97.1 5.381352.70 1574.81 1379.17 1604.00 Example 4 89.3 4.16 1349.52 1576.601352.07 1583.61 Example 5 89.9 4.76 1351.67 1577.88 1375.90 1603.70Comparative 80.2 4.05 1350.21 1576.28 1356.35 1582.31 Example 1Comparative 77.3 3.99 1349.25 1576.32 1361.48 1585.25 Example 2Comparative 79.5 3.32 1347.89 1578.39 1352.32 1580.15 Example 3Comparative 79.8 4.41 1351.58 1578.96 1351.51 1579.74 Example 4

Each carbonaceous electrode material was subjected to Raman spectrometryhaving a wavelength resolution of 4 cm⁻¹ using an argon laser with awavelength of 514.5 nm. Each carbonaceous electrode material was coatedwith silver having a thickness of 10 nm by a deposition process and wasthen tested by surface-enhanced Raman spectrometry as above. The valuesRG ratio, Pbb, Pba, Psb, and Pas of each sample were determined fromthese spectra. The results are shown in Table 1.

FIG. 2 is a graph of the relationship between the RG ratio and thecapacity recovery rate. The capacity recovery rate increases inproportion to the RG ratio. In particular, the capacity recovery rate ishigher than 85% at an RG ratio above 4.5. The results suggest that adecrease in capacity after storage in high temperature environments canbe suppressed.

FIG. 3 is a graph of the relationship between the Psb value and thecapacity recovery rate. The capacity recovery rate increases inproportion to the H_(sb) value. In particular, the capacity recoveryrate is higher than 85% at an H_(sb) value above 1365. The resultssuggest that a decrease in capacity after storage in high temperatureenvironments can be suppressed.

As described above, the ratio RG=Gs/Gb of the degree of graphitizationGs of the carbonaceous material, determined by a surface-enhanced Ramanspectrum, to the degree of graphitization Gb, determined by a Ramanspectrum measured using argon laser light, is limited to be at least 4.5in the present invention so as to suppress a decrease in the capacity ofa nonaqueous secondary battery using the carbonaceous material as anegative electrode component after storage in high-temperatureenvironments. Also, a carbonaceous material having a peak in awavelength range above 1,360 cm⁻¹ in a surface-enhanced Raman spectrumwhich is measured using an argon laser Raman spectrometer of awavelength of 514.5 nm may be used in the present invention so as tosuppress a decrease in the capacity of a nonaqueous secondary batteryusing the carbonaceous material as a negative electrode component afterstorage in high-temperature environments.

1. A method for making a negative electrode carbonaceous materialcomprising the steps of: (a) carbonizing a raw material, (b) heating theproduct of step (a) at a temperature of at most 200° , in the presenceof an oxidizer; and (c) graphitizing the product of step (b), wherein aratio RG=Gs/Gb of the product of step (c) is at least 4.5, wherein:Gb=Hbb/Hba; andGs=Hsb/Hsa; wherein: Hba is the height of a peak lying in a range of1,580 cm⁻¹ to 1,620 cm⁻¹ in a first Raman spectrum, said first Ramanspectrum being measured by means of an argon laser Raman spectrometer ofa wavelength of 514.5 nm and a wavelength resolution of 4 cm⁻¹; Hbb isthe height of a peak lying in a range of 1,350 cm−1 to 1,400 cm−1 in asecond Raman spectrum, said second Raman spectrum being measured bymeans of an argon laser Raman spectrometer of a wavelength of 514.5 nmand a wavelength resolution of 4 cm⁻¹; Hsa is the height of a peak lyingin a range of 1,580 cm⁻¹ to 1,620 cm⁻¹ in a first surface-enhanced Ramanspectrum, said first surface-enhanced Raman spectrum being measured bymeans of an argon laser Raman spectrometer of a wavelength of 514.5 nmand a wavelength resolution of 4 cm⁻¹ when silver having a thickness of10 nm is deposited on the product of step c.; and Hsb is the height of apeak lying in a range of 1,350 cm⁻¹ to 1,400 cm⁻¹ in a secondsurface-enhanced Raman spectrum, said second surface-enhanced Ramanspectrum being measured by means of an argon laser Raman spectrometer ofa wavelength of 514.5 nm and a wavelength resolution of 4 cm⁻¹ whensilver having a thickness of 10 nm is deposited on the product of stepc.
 2. A method for making a negative electrode carbonaceous material,comprising the steps of: (a′) carbonizing a raw material, (b′) heatingthe product of step (a′) to a temperature of at most 200° C. in thepresence of an oxidizer; and (c′) graphitizing the product of step (b′),wherein a surface-enhanced Raman spectrum of the product of step (c′)comprises a peak in a wavelength range above 1,360 cm⁻¹, saidsurface-enhanced Raman spectrum being measured by means of an argonlaser Raman spectrometer of a wavelength of 514.5 nm and a wavelengthresolution of 4 cm⁻¹ when silver having a thickness of 10 nm isdeposited on the product of step c′.